1. Field of the Invention
The present invention relates to a wireless LAN (Local Area Network) apparatus for use in a wireless LAN system.
2. Description of the Related Art
It has been possible to construct a wireless LAN system in a limited area so that data can be transmitted between a plurality of apparatuses via a wireless medium.
A system in an infrastructure mode which is essential according to the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard is composed of a wireless LAN access point (hereinafter referred to as an “AP”) and a plurality of wireless LAN apparatuses (stations: hereinafter referred to as “STAs”) residing within the radio wave range of the wireless LAN access point.
The STA handles data for each MAC service data unit (MSDU) or MAC management protocol data unit (MMPDU), which is a predetermined data unit. If the STA transmits a MAC service data unit (MSDU) or a MAC management protocol data unit (MMPDU) to a receiving apparatus, the transmission is carried out by fragmenting this data unit into MAC protocol data units (MPDU) that are MAC level frames each having a shorter data structure than the MSDU or MMPDU. This fragmentation is effective in improving the reliability of data transmission if channel characteristics are such that the transmission of long frames degrades the reliability of data transmission.
On receiving the fragments generated by the fragmentation, the STA inserts them into a queue. The queue temporarily stores data, process requests, and the like. Then, the plurality of fragments are reconstructed (defragmented) into the MSDU or MMPDU.
The STA supports the simultaneous reception of at least three data units (MSDUs or MMPDUs). Further, the STA comprises at least three reception counters for the respective data units being received. If the STA receives fragments constituting a data unit not managed by any reception counters, it discards all the fragments.
Each data unit is managed on the basis of a max receive lifetime indicative of the maximum allowable time required to receive one data unit. The reception counter starts counting on receiving the first fragment of the data unit. Then, if a reception timer exceeds the max receive time, the STA discards all the fragments of this data unit. If fragments succeeding this data unit are received after the max receive lifetime has been exceeded, the STA transmits an acknowledge (ACK) signal to the source apparatus to discard these succeeding fragments.
However, if a new data unit is received when the queue contains only a small amount of free areas, the free areas in the queue may be used up before all the fragments of this data unit are received. This may result in an overflow error.
On the other hand, the receiving STA uniformly discards the fragments of data units that have exceeded the max receive lifetime. Consequently, even the fragments of data units that have been almost completely defragmented may be discarded.